The big engine purred a little louder. Had an expert been there he would not have been able to detect a single alarming sound from the mechanism of the airship. For there was, in fact, little to go wrong. "Freezing up does not trouble me," Joe had explained as they were ascending, "for my radiator is cooled by paraffin, and you may expose that liquid to extremes of cold with little effect. Even if there were danger of its freezing, the explosions of the engine cause heat, which is absorbed by the paraffin, and I have taken steps, by throwing out of gear the cooling fan, to retain that heat. As for the rest, the same fluid passing through those lines of steel tubes to the motors overhead is constantly in action. The pressure applied to it tends to add to its temperature, so that there again we can defy the cold." The hum of the propeller told that the ship was in motion, for hitherto she had merely been ascending. Now the elevator screws were hardly rotating, while Dick and his friends could tell that they were advancing by the fact that the slopes of the mountain grew steadily nearer and nearer. The ship circled about the highest peak. She seemed to be looking for a landing-place. She even rested for a moment directly above the topmost pinnacle. And then Joe dropped her gently upon a smooth, level slope just beneath the summit. "All explorers plant flags to show what they have done," he cried, laughing at those gathered about him. "We will do the same. Come, half a dozen of us will be sufficient." They tore the door of the gallery open, for it was frozen fast, and struggled into the open, Joe and the Commander, with the Major, and Dick, and Alec, in close attendance. Bearing their oxygen cylinders strapped to their shoulders they trudged across the hard frozen snow, and within a few minutes had gained the summit. There they secured the staff of their Union Jack, pegged and roped it down, and promptly retraced their steps. "And now for a record," cried Joe. "I advise all of you to don gloves if you have them and to keep moving about. I mean to rush the rest of the distance." He covered more than half of his radiator, set the elevating motors buzzing, and then glanced anxiously at his barometer. They were rising, but very slowly. It seemed to take an endless age to get away from the peak they had just visited. The tiny Union Jack, looking forlorn amid the snow slopes, appeared as if it would keep them company for ever. "Turn that lever there," Joe commanded, pointing to one close to Dick's hand, for the midshipman was again in the engine-room. "If the outlet of my tank is frozen we shall have to halt for a while and apply heat. Ah, that's fortunate! Listen."